The UK's coronavirus alert level has been raised from a 3 to a 4, marking the first change in alert level since June.
The raise was recommended by the UK's four Chief Medical Officers, and means that there is now high or rapidly increasing transmission of coronavirus across the country.
What is the coronavirus alert system?
Coronavirus risk levels have been measured with a five point alert system since May, when the system was first unveiled by Prime Minister Boris Johnson.
Each stage indicates the level of threat that coronavirus currently poses to the country, and is used to decide how stringent social distancing measures to tackle the virus should be:
- Level 5 (red) - a "material risk of healthcare services being overwhelmed" - requiring extremely strict social distancing
- Level 4 - a high or rising level of transmission - social distancing should be enforced
- Level 3 - the virus is in general circulation - social distancing can be relaxed
- Level 2 - the number of cases and transmission are low - minimal social distancing
- Level 1 (green) - Covid-19 is no longer present in the UK - no social distancing required
How is the level of threat decided?
The threat level of coronavirus is determined by the Joint Biosecurity Centre (JBC), which looks at changes in infection rates using workplace, environmental and testing data.
The JBC also monitors local spikes of coronavirus with its "insight team", which advises local authorities and health officials.
Recommendations given by the JBC are reviewed and agreed by the four Chief Medical Officers of the UK before being officially announced.
Fundamentally, the alert level is determined by two factors:
- The reproduction (R) number of coronavirus, which measures how fast the virus is spreading
- The number of confirmed coronavirus cases at any one time
Why has the alert level been changed?
The alert level for coronavirus has been increased after a concerning spike in coronavirus cases observed across many parts of the UK.
In an address to the nation on Monday 21 September, Chief Scientific Adviser Sir Patrick Vallance and Chief Medical Officer Chris Whitty warned that the current spread of coronavirus could lead to 50,000 cases per day by mid-October if more stringent measures are not introduced.
Does a change in alert level mean more restrictions?
While a change in alert level isn't automatically tied to the introduction of stringent measures, the alert system is used by UK governments to decide how strict measures should be at any one time.
In England, restrictions are decided by the government's 'five tests'.
It is not yet clear whether the four nations of the UK will take an entirely devolved approach to any new restrictions.
Northern Ireland has already levied a ban on households mixing with one another as a result of rising cases over the past few weeks.